This invention generally relates to a fluidic actuating scheme, and more particularly, this invention relates to fluidic actuation for improved diffuser performance.
Typically, the maximum outlet to inlet area ratio of a gas turbine exhaust diffuser (and therefore the amount of effective flow diffusion following the last turbine stage) is constrained by flow separation issues and/or allowable axial diffuser length. Diffuser will exhibit separated flow if the expansion is too rapid (large diffuser angle) or the diffuser area ratio is too large.
For a given diffuser length, the area ratio is determined by the diffuser expansion angle. The maximum included angle, which can be tolerated before significant flow separation occurs, is generally of the order of 10 degrees. For diffusers that are not limited in length, the maximum area ratio that can be tolerated before significant separation occurs is generally of the order of 2.4 (outlet area divided by inlet area). For attached flow the pressure recovery is a function of area ratio and increases as area ratio increases. For turbine exhaust systems, any constraint on the exhaust diffuser area ratio imposes a limitation on maximum amount of work that can be extracted by the turbine.
A design that would allow larger diffusion angles without flow separation within the same or less axial length would provide larger areas ratios, improved pressure recovery, and increased gas turbine efficiency. For systems that already have acceptable pressure recovery the result could be significantly reduced diffuser length. Presently the exhaust diffusion system on an F-class gas turbine takes up approximately half of the overall gas turbine length.
Finally, diffuser performance as it relates to pressure recovery can be strongly affected by the diffuser inlet flow profile. For a typical F-class gas turbine the inlet flow profile varies as a function of machine load and amount of power produced. Turbine diffusers are designed to achieve the highest pressure recovery at full load operating conditions. At part load conditions, due to off-design inlet flow profiles and resulting flow separations, the pressure recovery of the diffuser can be degraded by a factor of 3.
Similarly, the performance of a steam turbine exhaust system is limited by geometry constraints and flow separation issues. For example, the down flow hood axial length cannot be increased without changing the bearing span of the machine rotor and the maximum area ratio allowable through the steam guide flow path, before flow separation occurs, yields a low value of the pressure recovery coefficient of 0.3 for the whole exhaust hood. For one type of an axial flow diffuser used in steam turbines, the maximum included angle that can be tolerated before significant separation (and losses) occurs is of the order of 10-15 degrees. This issue, in addition to constraints on the length of the diffuser, limits the exhaust pressure recovery coefficient to a value of 0.25-0.3.
Previously, options which have been identified to improve diffuser performance relative to conventional designs include use of splitter vanes, vortex generators and wall riblets. Splitter vanes have the disadvantage of increasing skin friction (and therefore losses) and appear to work relatively well only for uniform inlet flows. Inlet swirl, for instance, can substantially deteriorate the performance. Vortex generators and other passive devices need a high momentum core flow to re-energize the boundary layer and delay separation. In principle, they are expected to fail to yield a substantial increase in diffuser performance if, as it is the case downstream of the last turbine stage at the diffuser inlet, the diffuser inlet flow profile is severely skewed and characterized by large regions of low momentum fluid in the vicinity of the separation point. Evidence of diffuser performance improvement due to the use of ribs/riblets on the diverging diffuser walls is uncertain.